<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
    div{

        /* 
        设置box - sizing :border时
        给border padding 不会改变它的width ,height
         */
         box-sizing: border-box;

         border: 5px solid #333;
         padding: 10px;
         width: 100px;
         height: 100px;

         background: red;

    }
    
    </style>
</head>
<body>
    <div>

    </div>
</body>
</html>